The present invention relates to the field of semiconductor fabrication. In particular, it relates to the fabrication of field effect transistors (FETs), involving the formation of semiconductor materials of selected conductivity, carried on by implantation of dopants.
Fabrication of metal oxide semiconductor (MOS) FETs requires the formation of source and drain regions in a substrate of generally pure silicon (Si). The Si is provided in the form of a wafer, grown as a single crystal. Zones of the Si lattice are converted into regions of N or P conductivity by the addition of donor-type dopants, such as arsenic, for N regions, and acceptor-type dopants, such as boron, for P regions. These dopants are generally introduced by ion bombardment, in which ionized dopant atoms are energized and fired at the lattice, penetrating the crystal structure to a depth largely dependent on the bombardment energy and the ion mass.
It can be immediately gathered that such bombardment introduces crystal damage, in which lattice atoms are knocked out of lattice sites, while at the same time a certain number of the newly-introduced atoms will likewise come to rest in positions outside the lattice positions. Such out-of-position phenomena are termed defects. A vacant lattice site is termed a vacancy defect, while an atom located at a non-lattice site is referred to as an interstitial defect. The restorative method generally employed in the art consists of annealing the crystal, applying heat to the lattice to mildly energize the atoms, allowing them to work themselves back into the lattice structure, which provides the arrangement having the lowest overall energy level.